This is the renewal of the Women s and Infants Transmission Study at the University of Puerto Rico, Medical Sciences Campus. This application is for the fourth 5-year funding cycle. Accomplishments include the enrollment of the highest number of unique mother-infant pairs among the sites, the highest number of pregnant women and infants, the second highest number of deliveries, and a significant contribution of samples for the national or central WITS repository. Scientific accomplishments include the first comprehensive description of HIV disease progression in infants enrolled in the WITS; a consultative role in the CDC and PHS sponsored recommendations for the Prevention of Perinatal Transmission of HIV; CDC Disease Progression of Classification or staging for HIV infected infants; standardization and training of the neurological exam performed in WITS; and the reproductive choices of women enrolled in WITS. The WITS program in Puerto Rico has achieved a profound impact as a model program extended island wide for the prevention of perinatal transmission of HIV. Since September 1996 all infants born to women enrolled in the program have been free of HIV infection.